Circuit configuration for a brake system having an anti-lock control

ABSTRACT

The circuit configuration for a brake system having an anti-locking control generates pulse-type brake pressure control signals. For pressure rebuild-up after a pressure decrease, brake pressure is applied first at a steep and subsequently at a flatter gradient, this being achieved by a variable pulse (P1) and by short fixed pulses (P2) succeeding one another at a large interval. Circuits are provided rendering dependent the pulse and pulse break times (T 1 , T 2 , T k ) determining the pressure build-up 
     on the duration of the pressure build-up (T 1 ) during the steep-rise build-up in the preceding cycle, 
     on the duration (T 1  +nT 2 ) of the entire pressure build-up in the preceding cycle, and 
     on the duration (T o ) of the preceding pressure decrease, with the pulse times being so dimensioned 
     that, at a constant coefficient of friction and at a constant static pressure, the locking limit of the wheel is rereached after a predetermined period of time or after a predetermined pulse number. The duration of the variable and of the fixed pressure build-up pulses is computed from the pressure build-up of the preceding pressure or determined from a stored Table.

BACKGROUND OF THE INVENTION

The present invention relates to a circuit configuration for a brakesystem having an anti-locking control for controlling the pressurebuild-up upon commencement of the control by generating pulse-typecontrol signals capable to control multi-way valves that are insertedinto pressure-fluid conduits leading to the wheel brakes, with thepressure, by dimensioning the pulse and pulse break times, beingapplicable first at a steep and then at a flatter gradient, and with thepressure build-up, in the first cycle after commencement of the control,being dependent on the preceding pressure drop and, in the other cycles,on the pressure increase in the preceding cycles.

A circuit configuration of the afore-described type has already beentaught by DE-Patent 24 60 904 according to which the brake pressure,basically, during anti-locking control, first, is reincreased at a steepand, subsequently, at a flatter gradient. The steep pressure increase,preferably, is attained by an extended pulse while the flatter increaseis attained by a short pulse which two pulses succeed one another at arelatively large interval, with the duration of the steep-gradientpressure rise being varied in response to the pressure build-up in thepreceding control cycle(s). For dimensioning the steep phase of thefirst control cycle in respect of which there is no precedinglycontrolled pressure build-up, the level and duration, respectively, ofthe preceding brake pressure decrease will be taken into account.

According to European Patent Application No. 177 817, equally concernedwith the pressure reincrease first at a steep and subsequently at aflatter gradient, it is known to render dependent the pressure level ofthe first pressure build-up pulse determining the phase of the steeppressure rise, in addition, on a quotient resulting from the maximumwheel acceleration divided by the sum of the amounts of maximum wheelacceleration and maximum wheel deceleration. In one example, thepressure level of the first pressure build-up pulse corresponds to thepreceding pressure decrease multiplied by the above-mentioned quotientand a factor approximately "1".

SUMMARY OF THE INVENTION

A brake pressure control according to the foregoing will not, in allcases, bring about satisfactory results. It is, therefore, an object ofthe present invention to adjust, for attaining an improved control, thebrake pressure rebuild-up even more precisely to the respectivesituation and the respective conditions to insure, on the one hand, acontinued driving stability and, on the other hand, a short stoppingdistance through attaining, at an early stage, the anti-locking pressurelevel during the individual control cycles.

It has been found that this problem can be solved by a circuitconfiguration of the type described above, the special feature of whichresides in that circuits are provided rendering dependent the pulse andpulse break times determining the pressure build-up, in the cyclesfollowing the first cycle, on the duration of the pressure build-up orthe number of the pressure build-up pulses during the steep-gradientpressure build-up in the preceding cycle, on the duration of the entirepressure build-up phase of the preceding cycle and on the duration ofthe preceding pressure decrease, with the pulse times and especially theduration of the pressure increase in the steep-gradient phase being sodimensioned that at an approximately constant friction value andfriction coefficient, respectively, and at an approximately constantstatic pressure, the locking limit of the wheel is rereached after apredetermined period of time and a predetermined number of pulses.

Static pressure in this context means the brake pressure in the mastercylinder dependent on the level of force applied by the driver to thebrake pedal.

If the constant frictional coefficient requirement is not complied with,the locking limit is rereached sooner or only after a major number of"fixed" pulses, i.e. short pulses succeeding one another at a "fixed"interval.

The circuit according to the present invention, under all roadconditions, insures a rapid approach of the pressure to the optimumpressure level thereby precluding deceleration at an excessively lowrate, especially under unfavorable conditions, and attaining a shortstopping distance.

According to an advantageous form of the present invention, thesteep-gradient pressure build-up is attainable by a variable pulse whilethe flat-gradient pressure build-up is attainable by "fixed" pulses thatare relatively short pulses of a constant duration following one anotherat a constant interval. It is particularly advantageous to maintain,according to another form of the present invention, the duration ofthese so-called "fixed" pulses in proportion to the duration of thevariable pulses and the duration of the steep pressure rise,respectively.

The predetermined pulse number after which the wheel will rereach thelocking limit, advantageously, including the variable pulse, is betweenthree and five, preferably three pulses. Alternatively, it is alsopossible for the pulse times to be so dimensioned that a controlfrequency of about 3 to 10 Hz, preferably 3 to 5 Hz, is adjusted.

According to another form of the present invention, the duration of thesteep-gradient pressure rise can be determined in response to the numberof the pulses or the duration of the steep pressure rise in thepreceding cycle and in response to the entire pressure build-up phase orthe entire number of pulses according to computed or tabulized values.Also, it will be possible for the calculation to be based on the numberof the pulses of the flat-gradient pressure build-up phase in place ofthe entire number of pulses.

In a variety of cases, it is of advantage to adjust the value fordimensioning the steep pressure rise computed or determined from thestored Table, under consideration of the preceding pressure decrease,and to reduce or raise the same to a predetermined maximum or to apredetermined minimum value, respectively, in relation to the durationof the pressure decrease. The maximum value can be fixed to apredetermined value in the range between 40% and 60% while the minimumvalue can be fixed to a predetermined value in the range between 10% and30% of the duration of he preceding pressure decrease.

According to one form of the present invention, the determined value ofthe steep pressure increase is not raised upon reaching of the minimumvalue once the instantaneous frictional value is low (low frictionalvalue).

After raising of the steep pressure rise to the minimum value, therewill be no renewed raising in the following control cycles as long asthe duration of the pressure rise or the number of the pressure build-uppulses does not exceed a predetermined limit value of e.g. two pulsesper control cycle.

According to another form of the circuit configuration of the presentinvention, the duration of the steep pressure rise computed ordetermined from the Table can be raised in response to the previouslymeasured reacceleration if the same exceeds predetermined thresholdvalues.

According to another form of the present invention, the pressurebuild-up can already be initiated once the reacceleration exceeds apredetermined threshold value in the range between 10 and 20 g, e.g. 12to 15 g if, in the preceding cycle, the duration of the pressuredecrease was relatively extended and was above a predetermined thresholdvalue, respectively.

Further features, advantages and fields of application of the inventionwill become apparent from the following description with reference tothe enclosed circuit configuration, the diagrams and the Table concernedwith forms of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a circuit configuration according to thepresent invention,

FIG. 2 is a diagrammatic view of the pressure pattern during ananti-locking control, and of the corresponding valve pulses, and

FIG. 3 shows a Table for determining the duration of the steep pressurerise in response to values in the preceding cycle, which Table can bestored in the circuit configuration according to FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows the major electronic components of a circuit configurationaccording to the invention for a control channel, viz. the controlchannel of the left-hand front wheel VL. Some of the control channel ofthe right-hand font wheel VR is shown. For the sake of clarity, similarcontrol channels for the rear wheels HL and HR have been omitted fromFIG. 1.

As shown in FIG. 1, in the example illustrated, the rotating pattern ofwheels VL, VR (10, 10') measured with the aid of wheel sensors 1,1'(S_(VL), S_(VR)) In a following circuit 2 (2'), respectively a speedsignal v_(VL) (v_(VR)) suitable for further processing is obtained. Inanother circuit 3 (3'), the instantaneous slip lambda, the accelerationor deceleration (v), the jerk (v), in known manner, are measured fromthis signal, individually for each wheel under consideration of avehicle reference speed v_(REF).

To form this vehicle reference speed v_(REF), which is known to serve asa reference quantity for rating the instantaneous rotating pattern ofthe individual wheels, all wheel speed signals v_(VL), v_(VR), v_(HL)and v_(HR), in a circuit 4, are logically combined and analyzed byspecial criteria. The starting signal of circuit 4, viz. the v_(REF)signal, is then supplied to the individual circuits 3,3' since it isneeded, among others, for determining the slip lambda.

The processed sensor signals are analyzed in a control logic 5 (5'),with a so-called phase identification occurring to determine whether thewheel run is stable, a locking tendency has occurred, reacceleration iseffected etc. Also, conclusions are drawn from the analysis of knowncriteria as to the instantaneous coefficient of friction and roadconditions, respectively; in this respect, the coefficient of frictionidentification is relied upon. Under consideration of this information,finally, the pulse control signals are computed (pulse computation),through which the brake pressure increase and decrease of the desiredbrake pressure pattern in the wheel brakes of the individual wheels isto be controlled.

The control logic 5 (5'), in addition, is provided with a"reacceleration" circuit branch supplying signals once thereacceleration of the wheel VL (VR) exceeds predetermined peak values.As explained in greater detail hereinafter, the steep pressure increasephase is additionally prolonged at a particularly high reacceleration.

For the valve control, an additional circuit 6 is provided through whichthe pulse signals computed in the control logic 5 are modified, and bywhich, in response to additional criteria, the actual valve-excitingsignals are generated which, virtually, are fed to a final phase 7, viz.the "valve excitation".

These criteria, determined with the aid of a circuit 8 and considered bythe valve control 6 in fixing the valve exciting signals, among others,are derived from the measurement of the pressure decrease and thecorresponding pulse times in the preceding control cycle, respectively,from stored data on pressure decrease and pressure increase in thepreceding cycle and, finally, from predetermined threshold values forthe duration of the steep pressure increase phase. This will beexplained in greater detail hereinafter with reference to FIGS. 2 and 3.

Sensor 1,1' may be, for example, an inductive transducer having apassive or active measuring circuit generating a signal the frequency ofwhich is in proportion to the speed. The rest of the components of theschematic circuit according to FIG. 1, basically, can be realized byfirmly wired or program-controlled circuits, preferably usingmicrocomputers.

FIG. 2 shows the brake pressure pattern within a wheel brake during acontrolled brake operation, using the circuit configuration according tothe invention. The brake pressure control signals (i.e. pulse trains)insuring this pressure pattern, are also shown therein. Designated by SG(closed in de-energized condition) are those pulses that energize thebrake pressure decreasing valve, whereas SO (open in de-energizedcondition) refers to the switching position of the so-called inlet valvewhich, normally, is introduced into the pressure fluid conduit leadingfrom the brake pressure generator to the wheel brake. In de-energizedcondition, the SO-valve is switched to the passage position. The valvesare not shown therein.

In the operation herein described, the anti-locking control operationwill commence at time t₁. The SO-valve will be reswitched to theblocking position. By energizing the SG-valve during the time from t₂ tot₃, the brake pressure is decreased followed up to t₄ by a phase ofkeeping the pressure constant, wherein both valves SO and SG are closed.Once the wheel has re-entered a stable phase, the pressure isreincreased, first at a steep gradient during the period of time betweent₄ and T₅ (the duration of pulse P1 is T₁) and, subsequently, at a flatgradient (from t₅ to the end of pulse P2 at t₆) which is achieved by ashort valve opening pulse P2 succeeding one another at a relativelylarge distance T_(k). During the time T_(k) both valves SG and SO, againare switched to the blocking position, with the pressure fluid conduitpassing therethrough being closed. The duration T₁ of pulse P1 iscomputed by the control electronic unit and is variable, whereas theshort pulses P2 presently are designated as "fixed" pulses because thepulse interval T_(K) as well as duration T₂ in a multiplicity of casesare constant. However, according to an advantageous form of the presentinvention, the pulse duration T₂ is adjusted to the duration T₁ of thevariable pulse P1. T₂ in that case is always in proportion orapproximately in proportion to T₁.

In the control cycle commencing at t₄, the duration of the steep pulseP1 is determined in response to the duration T₀ of the pressuredecrease. In the control cycle commencing at t₁₄ and in all followingcontrol cycles, the first pulse and the variable pulse P1₁ respectively,orients itself by the preceding process of pressure increase, withduration T₁₁ of pulse P1₁ being so computed by the circuit configurationaccording to FIG. 1 as to enable the controlled wheel, at a constantcoefficient of friction and at a constant static pressure, to rereachthe stability limit after three pulses, the variable pulse included.Hence, the brake pressure, in a defined time, is again approximated tothe optimum value, thereby precluding deceleration at an excessively lowrate (which would result in an excessively slow pressure build-up) andadjusting a predetermined control frequency of, for example, 3 to 5 Hz.

In the present instance, the steep pressure increase is attained by asingle variable pulse. The duration T₁₁ of this pulse in the secondcontrol cycle (and in the following cycles) is computed from theduration T₁ of the steep pressure rise and the variable pulse P1 of thepreceding control cycle, respectively, and from the overall duration ofthe pressure buildup in the preceding control cycle, i.e. from pulse P1and the total number of the fixed pulses P2. It is of special advantageif predetermined quantities can be relied upon for determining theduration of this variable pulse P1₁, that are stored, in the controlcircuit, e.g. in the valve control 6 according to FIG. 1. Usingmicrocomputers, by such tabular access, the program duration will bereduced and the storage location requirements minimized. FIG. 3, by wayof example, shows an abstract from such a Table. The numerals identifythe units of time. The top row indicates the number of fixed pulsesP2_(n-1) in the preceding control cycle, whereas the first column,P1_(n-1) refers to the duration of the preceding variable pulse.

The duration of pulse P1_(n), hence, is directly conveyed by the matrix.According to one example of embodiment, the units of the Table wereabout 1 to 1.2 msec.

The value P1_(n) computed or taken from the Table, moreover, isdependent on the pressure decrease in the preceding cycle. Circuit 8causes the variable pulse P1₁ to have at least a predetermined length ascompared to the length of the preceding pressure decrease, e.g. at least20% of the pressure decrease duration T₀₁. The maximum duration of thevariable pulse P1, equally, is fixed by circuit 8. The maximum lengthand duration, respectively, of pulse P1 is limited to a predeterminedvalue in relation to the duration of the pressure decrease T₀, e.g. toabout 50% of the preceding pressure decreasing pulse.

This adjustment of or rise in the duration of the variable pulse P1through the threshold calculation in circuit 8 to the minimum value ofe.g. 20% of the preceding pressure decreasing pulse is, however,rendered ineffective once the frictional value identification in thecontrol logic 5 indicates a low friction value.

Moreover, the described embodiment of the invention provides blockingafter a rise in the pressure build-up and the variable pulse P1,respectively, to the minimum value, a renewed rise therein for one ormore cycles as long as occurrence of a predetermined number of build-uppulses in the pressure build-up phases is not exceeded: such ban on arise in the pressure build-up is lifted, for example, if more than twobuild-up pulses are computed in one cycle.

In addition, according to another embodiment of the invention, theduration T_(1n) of the variable pulse P_(1n) may be influenced inresponse to the reacceleration to which the corresponding wheel of thevehicle was exposed in the preceding cycle. If such a reaccelerationexceeds predetermined peak values, the duration of the variable pulseP_(1n) is increased by predetermined units of time.

According to one embodiment of the invention, the pressure rebuild-up isinitiated before the wheel has re-entered its stable phase. For thispurpose, the reacceleration again, is measured. Once it exceeds apredetermined threshold value which may be between 10 and 20 g, and oncethe pressure decrease in the preceding cycle was relatively extended,pressure is applied immediately, i.e. upon reaching of thereacceleration threshold, through one or several pulses. Once the wheelhas then rereached its stability limit, the pressure build-up iscontinued.

Consideration can also be given to additional criteria in thecomputation of the pressure build-up and in the length of the variablepulse P1, to take into account special conditions such as cornering,trouble, atypical wheel rotating pattern etc.

What is claimed:
 1. A circuit for controlling an anti-lock brakingsystem in which pressure in a brake cylinder associated with a wheel tobe braked is controlled by inlet and outlet valves in conduits leadingto and from the brake cylinder, said circuit comprising:means adaptedfor sensing the rotational behavior of said wheel to be braked and fordetecting:(a) a locking limit at which said wheel tends to lock, and (b)acceleration of said wheel; means adapted for supplying:(a) pressuredecrease pulses to said inlet and outlet valve means in conduits leadingto and from said brake cylinder associated with said wheel to conductfluid from said brake cylinder during the presence of said pressuredecrease pulses while fluid is not conducted to said brake cylinder todecrease pressure in said brake cylinder during the presence of saidpressure decrease pulses, and (b) pressure increase pulse trains to saidinlet and outlet valve means to repetitively conduct fluid to said brakecylinder during the presence of pulses of said pressure increase pulsetrains while fluid is not conducted from said brake cylinder to increasepressure in said brake cylinder during the presence of said pulses ofsaid pressure increase pulse trains, each of said pressure increasepulse trains having a first pulse and succeeding pulses; and means forcontrolling the durations, numbers, and spacings of said pulses of saidpressure increase pulse trains according to the following:(a) for aninitial pressure increase pulse train, dependent upon that pressuredecrease preceding said initial pressure increase pulse train, (b) for aparticular pressure increase pulse train following said initial pressureincrease pulse train, dependent upon that pressure increase precedingsaid particular pressure increase pulse train, (c) each said first pulseof said pressure increase pulse trains having a duration which isdependent upon the duration of said preceding pressure decrease and:(1)the maximum duration of each said first pulse of said pressure increasepulse trains is fixed between forty percent and sixty percent of saidduration of said preceding pressure decrease, and (2) the minimumduration of each said first pulse of said pressure increase pulse trainsis fixed between ten percent and thirty percent of said duration of saidpreceding pressure decrease, and (d) at an approximately constantcoefficient of friction and at an approximately constant staticpressure, said locking limit of said wheel is reached after apredetermined period of time.
 2. A circuit according to claim 1 whereinincreasing the duration of said first pulse in each of said pulse trainsis precluded when the duration falls below a predetermined minimum valuefor a low instantaneous coefficient of friction.
 3. A circuit accordingto claim 1 wherein after the duration of the first pulse in each of saidpulse trains is raised to a minimum value, in subsequent pressureincreases, renewed pressure increase is precluded if the duration of thepressure increase does not exceed a predetermined threshold.
 4. Acircuit according to claim 1 wherein after the duration of the firstpulse in each of said pulse trains is raised to a minimum value, insubsequent pressure increases, renewed pressure increase is precluded ifthe number of pulses does not exceed a predetermined threshold.
 5. Acircuit for controlling an anti-lock baking system in which pressure ina brake cylinder associated with a wheel to be braked is controlled byinlet and outlet valves in conduits leading to and from the brakecylinder, said circuit comprising:means adapted for sensing therotational behavior of said wheel to be braked and for detecting:(a) alocking limit at which said wheel tends to lock, and (b) acceleration ofsaid wheel; means adapted for supplying:(a) pressure decrease pulses tosaid inlet and outlet valve means in conduits leading to and from saidbrake cylinder associated with said wheel to conduct fluid from saidbrake cylinder during the presence of said pressure decrease pulseswhile fluid is not conducted to said brake cylinder to decrease pressurein said brake cylinder during the presence of said pressure decreasepulses, and (b) pressure increase pulse trains to said inlet and outletvalve means to repetitively conduct fluid to said brake cylinder duringthe presence of pulses of said pressure increase pulse trains whilefluid is not conducted from said brake cylinder to increase pressure insaid brake cylinder during the presence of said pulses of said pressureincrease pulse trains, each of said pressure increase pulse trainshaving a first pulse and succeeding pulses; and means for controllingthe durations, numbers, and spacings of said pulses of said pressureincrease pulse trains according to the following:(a) for an initialpressure increase pulse train, dependent upon that pressure decreasepreceding said initial pressure increase pulse train, (b) for aparticular pressure increase pulse train following said initial pressureincrease pulse train, dependent upon that pressure increase precedingsaid particular pressure increase pulse train, (c) each said first pulseof said pressure increase pulse trains having a duration which isdependent upon the duration of said preceding pressure decrease and:(1)the maximum duration of each said first pulse of said pressure increasepulse trains is fixed between forty percent and sixty percent of saidduration of said preceding pressure decrease, and (2) the minimumduration of each said first pulse of said pressure increase pulse trainsis fixed between ten percent and thirty percent of said duration of saidpreceding pressure decrease, and (d) at an approximately constantcoefficient of friction and at an approximately constant staticpressure, said locking limit of said wheel is reached after apredetermined number of pulses of said pressure increase pulse trains.6. A circuit according to claim 5 wherein increasing the duration ofsaid first pulse in each of said pulse trains is precluded when theduration falls below a predetermined minimum value for a lowinstantaneous coefficient of friction.
 7. A circuit according to claim 5wherein after the duration of the first pulse in each of said pulsetrains is raised to a minimum value, in subsequent pressure increases,renewed pressure increase is precluded if the duration of the pressureincrease does not exceed a predetermined threshold.
 8. A circuitaccording to claim 5 wherein after the duration of the first pulse ineach of said pulse trains is raised to a minimum value, in subsequentpressure increases, renewed pressure increase is precluded if the numberof pulses does not exceed a predetermined threshold.
 9. A circuit forcontrolling an anti-lock braking system in which pressure in a brakecylinder associated with a wheel to be braked is controlled by inlet andoutlet valves in conduits leading to and from the brake cylinder, saidcircuit comprising:means adapted for sensing the rotational behavior ofsaid wheel to be braked and for detecting:(a) a locking limit at whichsaid wheel tends to lock, and (b) acceleration of said wheel; meansadapted for supplying:(a) pressure decrease pulses to said inlet andoutlet valve means in conduits leading to and from said brake cylinderassociated with said wheel to conduct fluid from said brake cylinderduring the presence of said pressure decrease pulses while fluid is notconducted to said brake cylinder to decrease pressure in said brakecylinder during the presence of said pressure decrease pulses, and (b)pressure increase pulse trains having a frequency between 3 Hz and 10 Hzto said inlet and outlet valve means to repetitively conduct fluid tosaid brake cylinder during the presence of pulses of said pressureincrease pulse trains while fluid is not conducted form said brakecylinder to increase pressure in said brake cylinder during the presenceof said pulses of said pressure increase pulse trains, each of saidpressure increase pulse trains having:(1) a first pulse having aduration which is variable, and (2) succeeding pulses having fixeddurations and fixed spacings between said succeeding pulses; and meansfor controlling the durations, numbers, and spacings of said pulses ofsaid pressure increase pulse trains according to the following:(a) foran initial pressure increase pulse train, dependent upon that pressuredecrease preceding said initial pressure increase pulse train, (b) for aparticular pressure increase pulse train following said initial pressureincrease pulse train, dependent upon that pressure increase precedingsaid particular pressure increase pulse train, (c) at an approximatelyconstant coefficient of friction and at an approximately constant staticpressure, said locking limit of said wheel is reached after apredetermined period of time.
 10. A circuit for controlling an anti-lockbraking system in which pressure in a brake cylinder associated with awheel to be braked is controlled by inlet and outlet valves in conduitsleading to and from the brake cylinder, said circuit comprising:meansadapted for sensing the rotational behavior of said wheel to be brakedand for detecting:(a) a locking limit at which said wheel tends to lock,and (b) acceleration of said wheel; means adapted for supplying:(a)pressure decrease pulses to said inlet and outlet valve means inconduits leading to and from said brake cylinder associated with saidwheel to conduct fluid from said brake cylinder during the presence ofsaid pressure decrease pulses while fluid is not conducted to said brakecylinder to decrease pressure in said brake cylinder during the presenceof said pressure decrease pulses, and (b) pressure increase pulse trainshaving a frequency between 3 Hz and 10 Hz to said inlet and outlet valvemeans to repetitively conduct fluid to said brake cylinder during thepresence of pulses of said pressure increase pulse trains while fluid isnot conducted form said brake cylinder to increase pressure in saidbrake cylinder during the presence of said pulses of said pressureincrease pulse trains, each of said pressure increase pulse trainshaving:(1) a first pulse having a duration which is variable, and (2)succeeding pulses having fixed durations and fixed spacings between saidsucceeding pulses; and means for controlling the durations, numbers, andspacings of said pulses of said pressure increase pulse trains accordingto the following:(a) for an initial pressure increase pulse train,dependent upon that pressure decrease preceding said initial pressureincrease pulse train, (b) for a particular pressure increase pulse trainfollowing said initial pressure increase pulse train, dependent uponthat pressure increase preceding said particular pressure increase pulsetrain, (c) at an approximately constant coefficient of friction and atan approximately constant static pressure, said locking limit of saidwheel is reached after a predetermined number of pulses of said pressureincrease pulse trains.
 11. A circuit for controlling an anti-lockbraking system in which pressure in a brake cylinder associated with awheel to be braked is controlled by inlet and outlet valves in conduitsleading to and from the brake cylinder, said circuit comprising:meansadapted for sensing the rotational behavior of said wheel to be brakedand for detecting:(a) a locking limit at which said wheel tends to lock,and (b) acceleration of said wheel; means adapted for supplying:(a)pressure decrease pulses to said inlet and outlet valve means inconduits leading to and from said brake cylinder associated with saidwheel to conduct fluid from said brake cylinder during the presence ofsaid pressure decrease pulses while fluid is not conducted to said brakecylinder to decrease pressure in said brake cylinder during the presenceof said pressure decrease pulses, and (b) pressure increase pulse trainshaving a frequency between 3 Hz and 10 Hz to said inlet and outlet valvemeans to repetitively conduct fluid to said brake cylinder during thepresence of pulses of said pressure increase pulse trains while fluid isnot conducted from said brake cylinder to increase pressure in saidbrake cylinder during the presence of said pulses of said pressureincrease pulse trains, each of said pressure increase pulse trainshaving:(1) a first pulse having a duration which is variable, and (2)succeeding pulses having fixed durations and spacings between saidsucceeding pulses proportional to said duration of said first pulse ofsaid pressure increase pulse trains; and means for controlling thedurations, numbers, and spacings of said pulses of said pressureincrease pulse trains according to the following:(a) for an initialpressure increase pulse train, dependent upon that pressure decreasepreceding said initial pressure increase pulse train, (b) for aparticular pressure increase pulse train following said initial pressureincrease pulse train, dependent upon that pressure increase precedingsaid particular pressure increase pulse train, (c) at an approximatelyconstant coefficient of friction and at an approximately constant staticpressure, said locking limit of said wheel is reached after apredetermined period of time.
 12. A circuit for controlling an anti-lockbraking system in which pressure in a brake cylinder associated with awheel to be braked is controlled by inlet and outlet valves in conduitsleading to and from the brake cylinder, said circuit comprising:meansadapted for sensing the rotational behavior of said wheel to be brakedand for detecting:(a) a locking limit at which said wheel tends to lock,and (b) acceleration of said wheel; means adapted for supplying:(a)pressure decrease pulses to said inlet and outlet valve means inconduits leading to and from said brake cylinder associated with saidwheel to conduct fluid from said brake cylinder during the presence ofsaid pressure decrease pulses while fluid is not conducted to said brakecylinder to decrease pressure in said brake cylinder during the presenceof said pressure decrease pulses, and (b) pressure increase pulse trainshaving a frequency between 3 Hz and 10 Hz to said inlet and outlet valvemeans to repetitively conduct fluid to said brake cylinder during thepresence of pulses of said pressure increase pulse trains while fluid isnot conducted form said brake cylinder to increase pressure in saidbrake cylinder during the presence of said pulses of said pressureincrease pulse trains, each of said pressure increase pulse trainshaving:(1) a first pulse having a duration which is variable, and (2)succeeding pulses having fixed durations and spacings between saidsucceeding pulses proportional to said duration of said first pulse ofsaid pressure increase pulse trains; and means for controlling thedurations, numbers, and spacings of said pulses of said pressureincrease pulse trains according to the following:(a) for an initialpressure increase pulse train, dependent upon that pressure decreasepreceding said initial pressure increase pulse train, (b) for aparticular pressure increase pulse train following said initial pressureincrease pulse train, dependent upon that pressure increase precedingsaid particular pressure increase pulse train, and (c) at anapproximately constant coefficient of friction and at an approxiamtleyconstant static pressure, said locking limit of said wheel is reachedafter a predetermined number of pulses of said pressure increase pulsetrains.
 13. A circuit for controlling an anti-lock braking system inwhich pressure in a brake cylinder associated with a wheel to be brakedis controlled by inlet and outlet valves in conduits leading to and fromthe brake cylinder, said circuit comprising:means adapted for sensingthe rotational behavior of said wheel to be braked and for detecting:(a)a locking limit at which said wheel tends to lock, and (b) accelerationof said wheel; means adapted for supplying:(a) pressure decrease pulsesto said inlet and outlet valve means in conduits leading to and fromsaid brake cylinder associated with said wheel to conduct fluid fromsaid brake cylinder during the presence of said pressure decrease pulseswhile fluid is not conducted to said brake cylinder to decrease pressurein said brake cylinder during the presence of said pressure decreasepulses, and (b) pressure increase pulse trains to said inlet and outletvalve means to repetitively conduct fluid to said brake cylinder duringthe presence of pulses of said pressure increase pulse trains whilefluid is not conducted form said brake cylinder to increase pressure insaid brake cylinder during the presence of said pulses of said pressureincrease pulse trains, each of said pressure increase pulse trainshaving a first pulse and succeeding pulses; and means for controllingthe durations, numbers, and spacings of said pulses of said pressureincrease pulse trains according to the following:(a) for an initialpressure increase pulse train, dependent upon that pressure decreasepreceding said initial pressure increase pulse train, (b) for aparticular pressure increase pulse train following said initial pressureincrease pulse train, dependent upon that pressure increase precedingsaid particular pressure increase pulse train, (c) each said first pulseof said pressure increase pulse trains having a duration which isdependent upon a previously measured acceleration of said wheel oncesaid acceleration exceeds a predetermined threshold value, and (d) at anapproximately constant coefficient of friction and at an approximatelyconstant static pressure, said locking limit of said wheel is reachedafter a predetermined period of time.
 14. A circuit for controlling ananti-lock braking system in which pressure in a brake cylinderassociated with a wheel to be braked is controlled by inlet and outletvalves in conduits leading to and from the brake cylinder, said circuitcomprising:means adapted for sensing the rotational behavior of saidwheel to be braked and for detecting:(a) a locking limit at which saidwheel tends to lock, and (b) acceleration of said wheel; means adaptedfor supplying:(a) pressure decrease pulses to said inlet and outletvalve means in conduits leading to and from said brake cylinderassociated with said wheel to conduct fluid from said brake cylinderduring the presence of said pressure decrease pulses while fluid is notconducted to said brake cylinder to decrease pressure in said brakecylinder during the presence of said pressure decrease pulses, and (b)pressure increase pulse trains to said inlet and outlet valve means torepetitively conduct fluid to said brake cylinder during the presence ofpulses of said pressure increase pulse trains while fluid is notconducted from said brake cylinder to increase pressure in said brakecylinder during the presence of said pulses of said pressure increasepulse trains, each of said pressure increase pulse trains having a firstpulse and succeeding pulses; and means for controlling the durations,numbers, and spacings of said pulses of said pressure increase pulsetrains according to the following:(a) for an initial pressure increasepulse train, dependent upon that pressure decrease preceding saidinitial pressure increase pulse train, (b) for a particular pressureincrease pulse train following said initial pressure increase pulsetrain, dependent upon that pressure increase preceding said particularpressure increase pulse train, (c) each said first pulse of saidpressure increase pulse trains having a duration which is dependent upona previously measured acceleration of said wheel once said accelerationexceeds a predetermined threshold value, and (d) at an approximatelyconstant coefficient of friction and at an approxiamtley constant staticpressure, said locking limit of said wheel is reached after apredetermined number of pulses of said pressure increase pulse trains.15. A circuit for controlling an anti-lock braking system in whichpressure in a brake cylinder associated with a wheel to be braked iscontrolled by inlet and outlet valves in conduits leading to and fromthe brake cylinder, said circuit comprising:means adapted for sensingthe rotational behavior of said wheel to be braked and for detecting:(a)a locking limit at which said wheel tends to lock, and (b) accelerationof said wheel; means adapted for supplying:(a) pressure decrease pulsesto said inlet and outlet valve means in conduits leading to and fromsaid brake cylinder associated with said wheel to conduct fluid fromsaid brake cylinder during the presence of said pressure decrease pulseswhile fluid is not conducted to said brake cylinder to decrease pressurein said brake cylinder during the presence of said pressure decreasepulses, and (b) pressure increase pulse trains to said inlet and outletvalve means to repetitively conduct fluid to said brake cylinder duringthe presence of pulses of said pressure increase pulse trains whilefluid is not conducted from said brake cylinder to increase pressure insaid brake cylinder during the presence of said pulses of said pressureincrease pulse trains, each of said pressure increase pulse trainshaving a first pulse and succeeding pulses; and means for controllingthe durations, numbers, and spacings of said pulses of said pressureincrease pulse trains according to the following:(a) for an initialpressure increase pulse train, dependent upon that pressure decreasepreceding said initial pressure increase pulse train, (b) for aparticular pressure increase pulse train following said initial pressureincrease pulse train, dependent upon that pressure increase precedingsaid particular pressure increase pulse train, and (c) at anapproximately constant coefficient of friction and at an approximatelyconstant static pressure, said locking limit of said wheel is reachedafter a predetermined period of time; and means for initiating pressureincrease once acceleration exceeds a predetermined threshold valuebetween 10 g and 20 g if the duration of said preceding pressuredecrease was extended and above a predetermined threshold value.
 16. Acircuit for controlling an anti-lock braking system in which pressure ina brake cylinder associated with a wheel to be braked is controlled byinlet and outlet valves in conduits leading to and from the brakecylinder, said circuit comprising:means adapted for sensing therotational behavior of said wheel to be braked and for detecting:(a) alocking limit at which said wheel tends to lock, and (b) acceleration ofsaid wheel; means adapted for supplying:(a) pressure decrease pulses tosaid inlet and outlet valve means in conduits leading to and from saidbrake cylinder associated with said wheel to conduct fluid from saidbrake cylinder during the presence of said pressure decrease pulseswhile fluid is not conducted to said brake cylinder to decrease pressurein said brake cylinder during the presence of said pressure decreasepulses, and (b) pressure increase pulse trains to said inlet and outletvalve means to repetitively conduct fluid to said brake cylinder duringthe presence of pulses of said pressure increase pulse trains whilefluid is not conducted from said brake cylinder to increase pressure insaid brake cylinder during the presence of said pulses of said pressureincrease pulse trains, each of said pressure increase pulse trainshaving a first pulse and succeeding pulses; and means for controllingthe durations, numbers, and spacings of said pulses of said pressureincrease pulse trains according to the following:(a) for an initialpressure increase pulse train, dependent upon that pressure decreasepreceding said initial pressure increase pulse train, (b) for aparticular pressure increase pulse train following said initial pressureincrease pulse train, dependent upon that pressure increase precedingsaid particular pressure increase pulse train, and (c) at anapproximately constant coefficient of friction and at an approximatelyconstant static pressure, said locking limit of said wheel is reachedafter a predetermined number of pulses of said pressure increase pulsetrains; and means for initiating pressure increase once accelerationexceeds a predetermined threshold value between 10 g and 20 g if theduration of said preceding pressure decrease was extended and above apredetermined threshold value.